LEDs are optoelectronic devices, which emit light by recombining injected electrons and holes radiatively. Depending on a bandgap of active material in a particular optoelectronic device, LEDs can emit at a wide range of wavelengths from ultraviolet to infrared. However, the wavelengths of light which are of major interest are those in the visible region. LEDs emitting in the visible spectrum (typically from ˜400 nm (purple) to ˜700 nm (red)) are visible to the human eye and are thus useful for illumination purposes. LEDs that emit visible light are also useful for providing visible indicators.
In order to emit light at visible wavelengths, many LEDs are constructed using elements from group III and V of the Periodic Table. Three (3) of these elements are gallium (Ga), indium (In) and nitrogen (N). Such materials are doped with “impurities,” small quantities of materials selected from other columns of the periodic table, to allow electrical activity, which in turn generates light via the recombination of an electron from a conducting state to a valence state.
The devices above are referred to as being of the (In,Ga) N material group. LEDs fabricated from this group of materials include monochromatic LEDs, which emit with single spectral peak and a narrow linewidth (e.g., ˜30 nm). LEDs fabricated using the (In,Ga) N material system can be made to emit monochromatic light ranging from ˜380 nm (near UV) to ˜580 nm (i.e., green) by changing the indium composition in the material system. Monochromatic LEDs, are often used as light indicators, where only a single color is required.
Pure white light, on the other hand, is broadband, i.e., a polychromatic light. It cannot be generated directly with a single LED. However, if an LED can be made to generate light at several discrete wavelengths or several relatively continuous bands of wavelengths, the resultant spectrum may nevertheless be considered polychromatic and the light emitted from such an LED will appear to the human eye to be white.
For illumination purposes, white light is generally preferred over non-white light. As lighting devices, LEDs are superior to incandescent lamps and fluorescent tubes, in terms of luminous efficiency, lifetime, robustness and environmental friendliness.
Currently, there are two major or principal methods of making broadband LED light sources. The first method makes use of phosphors for “color down” conversion. Phosphorescent materials that emit light when exposed to certain wavelengths of radiation are traditionally used for color conversion in light-emitting diodes (LEDs). A device may emit a high-energy photon, and the phosphor may absorb it and then re-emit a lower-energy and thus differently colored photon.
Such phosphors absorb shorter-wavelength photons and re-emit longer wavelength photons. For white light emission, green and red light-emitting phosphors may be used. It should be observed that any form of color conversion involves energy losses. While green phosphors may have quantum efficiencies of up to 90%, quantum efficiencies of red phosphors are typically limited to around 40%. This, in turn, translates to low wall-plug efficiency.
In such color down conversion schemes, a shorter wavelength monochromatic LED, such as an InGaN LED emitting at 460 nm (blue), may be used as a excitation light source. Such light may be used to excite luminescence in phosphors emitting at longer wavelengths, such as green and red. A resultant light is comprised of components from different parts of the visible spectrum, and is thus considered broadband light. Since the phosphor particles are small (e.g., on a nanometer scale) and indistinguishable to the naked eye, the emitted light appears as white if the proportions of the different colors are right. This form of white light generation is similar to that employed in fluorescent tubes.
However, there are many drawbacks associated with phosphors, including limited lifetime, Stokes-wave energy loss, low reliability and low luminous efficiency.
Another method of making a broadband LED light source is to mount several discrete LED chips onto a single package, each of which emits a different color. These devices are often called multi-chip LEDs, where LEDs emitting at the primary colors of light (i.e., blue, green and red) are mounted onto a single package. However, true “white” light emission cannot be achieved using this technique. Each LED chip is typically over 100 microns in dimension, while the separation of LED chips is of the same order. As a result, the colors are not homogenized and therefore appear as discrete colors to the naked eye unless placed at very far distances, by which time an LED's intensity has dropped immensely.